Hi Mark,
Benfleet is outside zones 1-6 so it’s not possible to combine that with another destination outside zones 1-6. Your best option is to get an Amersham to zones 1-6 travelcard which is routed “AAA zones 7-9” and use that with a Benfleet to boundary zone 6 return. If you can be certain that both journeys will be via Upminster then you can buy Benfleet to Upminster which can be bought online. The boundary zone ticket cannot be bought online.